Method of blast furnace operation utilizing selective recycling of peripheral gas stream

ABSTRACT

There is provided a method of blast furnace operation utilizing selective recycling of peripheral gas stream wherein the number of outlet ports for peripheral gas stream and the position of the gas outlet ports relative to the inlet ports for reducing gas are optimalized and the content of nitrogen in the recycling gas and the content of nitrogen in the reducing gas to be injected are controlled within the respective predetermined ranges.

BACKGROUND OF THE INVENTION

The present invention relates to a method of blast furnace operationutilizing selective recycling of peripheral gas stream wherein thenumber of outlet ports for peripheral gas stream and the position of thegas outlet ports relative to the inlet ports for reducing gas areoptimalized and the content of nitrogen in the recycling gas and thecontent of nitrogen in the reducing gas to be injected are controlledwithin the respective predetermined ranges, whereby a blast furnaceoperation is ensured which is highly efficient and stable. (According tothe present invention, what is termed as the peripheral gas stream isdefined as a gas stream that flows in an area between the inner wallsurface of a blast furnace and a seven-tenths point on the radiusextending from the cross-sectional center of the blast furnace to thefurnace wall.)

Many different types of recycling processes are known in the art inwhich the gas taken out from the top of the furnace or the furnace shaftand reformed into a gas suitable for reducing purposes after thenecessary treatment, is injected into the blast furnace at the lowerlevel thereof. They include, for example, (a) Wiberg process, (b)Purofer process, (c) the process disclosed in the published JapanesePatent Application (Publication No. 27127/71), (d) Bureau of Minesexperiment and (e) Nixon process. However, though the processes of (a)through (c) above utilize the recycling of the furnace gas, the reducinggas is injected into the blast furnace through the ordinary tuyeres andtherefore the injection of the gases is effected at a single level.Therefore, these processes do not belong to the prior art techniques towhich the improvements according to the present invention pertain, sincethere is no big difference in nitrogen content between the compositionof the gas in the central area of the furnace and that of the gas in theperipheral area of the furnace. As compared with these processes, thoseof (d) and (e) above are more closely related to the techniques to whichthe subject matter of the present invention pertains in that theinjection of the reformed reducing gas is effected through inlet portsother than the ordinary tuyeres and that two injection levels areutilized. However, in the Bureau of Mines experiment of (d), the wastegas mixed up irrespective of whether the peripheral gas stream orcentral gas stream, is removed and recycled for the purpose ofregulating the temperature of the reducing gas to be injected. On theother hand, the Nixon process of (e) is designed so that the wholequantity of the waste gas is passed through a purifier and a portion ofthe purified gas is recycled. In these processes, therefore, theso-called peripheral gas stream flowing in the peripheral area of thefurnace top or the furnace shaft is not selectively recycled.

In general, while nitrogen contained in the furnace gases serves as acarrier of heat in the process of heat exchange between the gases andthe charge, it is inert in the reduction of ore. Therefore, it isdesirable that the nitrogen content of the reducing gas should beminimized so far as the heat exchange between the gases and the chargedoes not have any detrimental effect on the operation of the furnace.Accordingly, it is essential for improved rate of operation andreduction in fuel ratio to minimize the introduction of nitrogen in theinjected gas. In the above-mentioned recycling processes using two levelinjection, however, a gas mainly consisting of air and containing agreater quantity of nitrogen is injected through the ordinary tuyeres atthe lower injection level, while a reducing gas having a low nitrogencontent is injected through the upper level inlet ports. Consequently,these gases do not completely mix with each other with the result thatin the furnace section above the upper level gas inlet ports, the highnitrogen content gas predominates the central area and the low nitrogencontent gas predominates the peripheral area. A method of blast furnaceoperation in which such a low nitrogen content peripheral gas stream isselectively recycled and utilized as a reforming source to produce areformed reducing gas that suits the desired blast furnace operation,has been proposed in the published Japenese Pat. Application(Publication No. 49671/71) of the Applicant.

The inventors have conducted various researches in the methods of blastfurnace operation which utilize recycling of the abovedescribedperipheral gas stream and have discovered that in practising thisrecycling of peripheral gas stream, there is a certain optimum rangewith respect to the number of outlet ports for the peripheral gas streamas well as the angular relation between the peripheral gas outlet portsand the inlet ports for reducing gas. Further, it has been found that bycontrolling the nitrogen content of the peripheral gas to be taken outand that of the reducing gas to be injected within respectivepredetermined ranges, a satisfactory and stable blast furnace operationcould be ensured.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of blastfurnace operation of the type in which a reducing gas produced byreforming a mixture of a peripheral gas stream taken out from the top orshaft of a blast furnace and a fresh feed gas is injected into thefurnace through reducing gas inlet ports other than the ordinarytuyeres, wherein the peripheral gas stream is taken out through aplurality of outlet ports whose number is determined in accordance withthe following equations:

when ##EQU1## and when ##EQU2## where ##EQU3## U/U+V = ratio of injectedreducing gas quantity (U) to the total gas quantity (U + V) passingthrough the furnace

V = quantity of gas (Nm³ /hr) produced from the blast by the combustionof coke and solution loss reaction

m = number of outlet ports for peripheral gas stream

n = number of inlet ports for reducing gas

C/h = ratio by weight of C to H in a fresh feed gas and wherein theoutlet ports for peripheral gas stream are positioned relative to theinlet ports for reducing gas in such a manner that there is apredetermined angular relation therebetween established in accordancewith the following equation: ##EQU4## where θ = angle made betweenvertical plane formed by peripheral gas stream outlet port and furnacecenter line and vertical plane formed by reducing gas inlet port andfurnace center line (in degrees)

n = number of reducing gas inlet ports

U = injected reducing gas quantity (Nm³ /hr)

V = gas quantity (Nm³ /hr) produced from the blast by the combustion ofcoke and solution loss reaction

It is another object of the present invention to provide a method ofblast furnace operation of the above type wherein in operating the blastfurnace with the reducing gas inlet ports numbered and positioned in themanner described above, the value x of % N₂ in the gas taken out fromthe peripheral area of the furnace is controlled within a range given bythe following equation: ##EQU5## where

    0.1 ≦ U/(U+V) ≦ 0.9

and the value y of % N₂ in the reducing gas injected into the furnace iscontrolled within a range given by the following equation: ##EQU6##where

    0.1 ≦ U/(U+V) ≦ 0.9.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a is a diagram showing the separate type reducing gas distributionin a blast furnace provided with 8 reducing gas inlet ports.

FIG. 1b is a diagram showing the annular reducing gas distribution in ablast furnace provided with 24 (twenty four) reducing gas inlet ports.

FIG. 2 is a graph showing the relationship between the number ofreducing gas inlet ports and the ratio of the injected reducing gasquantity to the total gas quantity in the furnace, U/(U+V).

FIG. 3 is a schematic diagram of a blast furnace showing an exemplaryangular relation between an outlet port for peripheral gas stream and aninlet port for reducing gas.

FIG. 4 is a graph showing an allowable range of angle θ for the positionof the peripheral gas outlet ports.

FIG. 5 is a graph showing an optimum range of % N₂

( x ) contained in the gas taken out from the furnace.

FIG. 6 is a graph showing an optimum range of % of N₂

(y) contained in the reducing gas to be injected.

FIG. 7 is a schematic diagram of an exemplary blast furnace plant usedfor performing the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in greater detail withreference to the accompanying drawing and the illustrated examples.

The distribution of a reducing gas injected into a blast furnace dependson the quantity of the reducing gas injected and the number of the inletports used. For example, in FIG. 1 showing a case where the ratio of theinjected reducing gas to the total quantity of the gas passing through ablast furnace, U/(U+V) is 0.4, the distribution of the reducing gas inthe upper section of a furnace shaft provided with 8 inlet ports isshown in FIG. 1a, and the similar distribution of the reducing gas in ablast shaft having 24 inlet ports is shown in FIG. 1b. In the case ofFIG. 1a, the distribution of the injected reducing gas consists of 8(eight) separate circles about the respective inlet ports. In the caseof FIG. 1b involving 24 inlet ports, the reducing gases injected throughthe respective inlet ports are joined to form an annular distribution.With these distributions, it will be seen from the separate typereducing gas distribution of FIG. 1a that the reducing gas just abovethe inlet ports has the highest concentration and the lowest nitrogencontent. Therefore, to obtain the recycling gas having a low nitrogencontent, the gas near the portion of the charge right above the reducinggas inlet ports must be taken out. On the other hand, the annulardistribution of FIG. 1b indicates that the concentration of the reducinggas increases and its value of % N₂ decreases in proportion to thedistance from the center of the blast furnace. Thus, to obtain therecycling gas with a low value of % N₂, it is desirable that the gas inthe peripheral area of the furnace be taken out and that the number ofoutlet ports for the peripheral gas be greater than a certain number.

On the other hand, CO₂ and H₂ O contained in the taken-out top gas fromthe peripheral area of the furance react with the hydrocarbon in a freshfeed gas and produces a reducing gas having higher CO and H₂ contentswhich is injected into the blast furnace. As a result, the recycling gasquantity Ur taken out from the peripheral area varies depending on theinjected reducing gas quantity U and it also varies depending on thecomposition of a fresh feed gas to be mixed with the recycling gasduring reforming process. As the result of various researches, theinventors have found that in consideration of quality requirements ofreducing gas, the taken-out gas quantity Ur is determined so that thefollowing equation which shows the relations of the taken-out gasquantity Ur with the injected quantity U of reducing gas and the ratioC/H (weight ratio) of fresh feed gas is satisfied: ##EQU7## If the flowof the taken-out recycling gas is too small as compared with thatdetermined by the above equation, an increased amount of soot will beproduced in the reducing gas, whereas an excessive amount of thetaken-out recycling gas will increase the amounts of CO₂ and H₂ O in thereducing gas.

Within the furnace, the injected reducing gas occupies an area whichcorresponds to the ratio of the injected reducing gas to the total gasquantity, i.e., U/(U+V). For example, if U/(U+V) = 0.4 (FIG. 1), thenthe area determined by the line of 0.5 mole fraction of the reducing gasinjected into the furnace approximately corresponds to the ratioU/(U+V), and the concentration of the gas sharply differs between insideand outside of said line. As a result, in order to remove the recyclinggas having a low N₂ content, it is necessary to take out the recyclinggas from the gas stream occupying the area above the inlet ports (in thecase of the separate type distribution) or the peripheral area (in thecase of the annular distribution) with the ratio of U/(U+V). In the caseof the separate type distribution, for example, if inlet ports numberingn are used to inject the quantity U, then the flow quantity of reducinggas per inlet port is U/n and thus the maximum quantity of the recyclinggas available through each outlet port is U/n. On the other hand, if thetotal quantity of recycling gas to be taken out is determined inaccordance with equation (1), then the number m of outlet ports is givenby the following equation:

    m ≧ Ur/(U/n) (2) from equations (1) and (2), we obtain ##EQU8##

Whether the distribution of the reducing gas within the furnace takesthe form of a separate type distribution or annular distribution dependson the ratio of the injected reducing gas quantity to the total gasquantity in the furnace, U/(U+V) and the number n of the inlet portsused (FIG. 2). Let k represent the critical number of inlet ports, thenthe value of k is given by the following equation: ##EQU9## In otherwords, if the number of inlet ports is smaller than k, then thedistribution of the reducing gas takes a separate form, whereas if it isgreater than k, the distribution of the reducing gas takes an annularform. And when computed on the basis of k inlet ports, the number m ofoutlet ports required for producing a quality reducing gas is given fromequation (3), as follows: ##EQU10##

While the use of inlet ports greater than k results in an annulardistribution of the injected reducing gas, no further increase in thenumber of the inlet ports can change this distribution form. Therefore,in the case of the annular distribution (provided that the number of theinlet ports is greater than k), the number of outlet ports must bedetermined, independently of the number of the inlet ports, to satisfyequation (5) to obtain a quality reducing gas.

If follows from the foregoing that the recycling gas suitable forproducing quality reducing gas having a low N₂ content can be alwaysobtained if the following condition is satisfied:

If n ≦ k (when the distribution of the reducing gas is a separatedistribution), the number m of outlet ports for the peripheral gasstream is given as ##EQU11## or if n>k (when the distribution of thereducing gas is an annular distribution), the number m of outlet portsfor the peripheral gas stream is given as ##EQU12##

It has also been discovered by the inventors that even in cases wherethe gas removed through a number of peripheral gas outlet portsdetermined in the manner described above was recycled, the quality ofthe gas thus recycled differed depending on the angular relation betweenthe outlet ports and the inlet ports for reducing gas and that theperipheral gas with improved quality could be taken out by angularlyadjusting the relative positions of these inlet and outlet ports. It hasbeen additionally discovered that if the nitrogen content of theperipheral gas to be taken out and that of the reducing gas to beinjected were respectively controlled within its predetermined range, astable and improved blast furnace operation could be ensured.

Generally, the injected reducing gas occupies the peripheral area of thefurnace and the gas produced from the blast occupies the central area ofthe furnace. However, since there occurs mixing by diffusion betweenthese gases, the central gas stream having a higher N₂ content is mixedto some extent with the peripheral gas stream of lower N₂ content. Wherethe amount of the injected reducing gas is small, this lowers the effectof reducing N₂ content due to the taking-out of the peripheral gasstream. Accordingly, it is desirable that the ratio of the injectedreducing gas quantity to the total gas quantity in the furnace, U/(U+V)is greater than 0.1.

On the other hand, the reforming process for producing a reducing gasusually utilizes a reaction that results in an increased volume such asshown below:

    CO.sub.2 + CH.sub.4 → 2CO + 2H.sub.2

The required CO₂ is supplied from the recycled top gas and CH₄ iscontained in a fresh feed gas, e.g., coke-oven gas fed to a reformerfurnace. The produced CO and H₂ are introduced into the blast furnace asreducing gases. Since the amount of the recycling gas used for reformingprocess is smaller than the amount of the injected reducing gas, aportion of the injected reducing gas is utilized as the recycling gasand the remainder leaves the furnace as waste gas. As the flow of thereducing gas increases, the ratio of the reducing gas in the central gasstream increases similarly as in the peripheral gas stream withresultant decrease in the N₂ content of the central gas stream. If thereducing gas is injected excessively, the difference in N₂ contentbetween the central gas stream and the peripheral gas stream decreasesand hence the beneficial effect of taking out the peripheral gas streamdeteriorates. Therefore, it is desirable that the ratio U/(U+V) bemaintained below 0.9.

The composition of the peripheral gas stream is influenced by theposition of their outlets on the furnace, and the position of outletports through which the peripheral gas of the desired properties can betaken out lies within a range of angles relative to the position of theinlet ports for reducing gas and the center of the furnace. In otherwords, as shown in FIG. 3, if lines X and X' are respectively drawn fromthe center A of the outlet port and the center B of the inlet porttoward the center line Y of the blast furnace to produce two imaginaryvertical planes including respectively the sides Y and X and the sides Yand X', in accordance with the finding by the inventors, the peripheralgas stream having the lowest % N₂ can be taken out by adjusting thevalue of an angle θ formed by the two vertical planes within the rangedetermined by the following equation (6): ##EQU13## where θ is indegrees

U = quantity of reducing gas injected

n = number of inlet ports for reducing gas

V = quantity of gas produced from the blast by

the combustion of coke and solution loss reaction

The allowable limits of the angle θ according to equation (6) are shownin the graph of FIG. 4, and it is essential that the value of θ iscontrolled and maintained within the range indicated by the hatched areain FIG. 4.

The proper position of the peripheral gas outlet ports in the directionof the height of the blast furnace is in the area between the linehigher than 1/3 of the shaft height and near the stock line.

Further, as will be seen from equation (6), the angle θ is considerablyaffected by the ratio of the injected reducing gas quantity (U) to thetotal gas quantity (U+V) and therefore it is desirable that in order totake out the peripheral gas of the desired properties and use iteffectively in the operation of the blast furnace, variation in thecomposition of the injected reducing gas, particularly the variation ofN₂ content which is detrimental to the reaction within the furnace isminimized to ensure stable operating conditions of the furnace. Thecomposition of the taken-out recycling gas and that of a reducing gas tobe injected mutually affect each other and thus any change in thecomposition of one causes a change in the composition of the other.Consequently, in order to ensure stable operating conditions of thefurnace, it is desirable that in addition to taking-out and recyclingthe peripheral gas stream through a proper number of properly positionedoutlet ports, the value of % N₂ (x ) in the taken-out peripheral gasstream and the value of % N₂ (y) in a reducing gas to be injected becontrolled respectively within the ranges given by the followingequations (7) and (8): ##EQU14## where

x = % N₂ contained in taken-out peripheral gas stream

y = % N₂ contained in reducing gas injected

U = quantity of reducing gas injected

V = quantity of gas produced from the blast by the combustion of cokeand solution loss reaction.

Figs. 5 and 6 show the graphic representations of the above equations(7) and (8), and the hatched portions represent the respective ranges ofvalues to which x and y are to be adjusted.

The following examples describe in greater detail the operation of anexemplary blast furnace plant shown in FIG. 7 in accordance with thenovel method of this invention.

Briefly, the blast furnace plant of FIG. 7 was operated as follows. Aperipheral gas stream 3 in the upper section of the shaft of a blastfurnace 5 was conducted to a demister and dust catcher 7 and the gas 3freed from dust and water was then preheated up to 1,300°C by apreheater 8. On the other hand, a coke-oven gas 4 as a fresh feed gaswas passed to a preheater 9 where it was preheated up to 900°C. The thuspreheated gases 3 and 4 were then mixed together and reformed in areformer furnace 6 and the reformed gas was introduced as a reducing gas1 into the blast furnace 5 at positions above the tuyeres for an airblast 2. In FIG. 7, numeral 10 designates the top gas which was notrecycled.

EXAMPLE 1:

The blast furnace plant shown in FIG. 7 was operated utilizing differentnumbers of outlet ports for peripheral gas stream. The following Table 1shows the comparison of the blast furnace operation according to themethod of this invention and according to the conventional methods.

                  Table 1                                                         ______________________________________                                                   Separate type                                                                             Annular reducing                                                  reducing gas                                                                              gas                                                               distribution                                                                              distribution                                                          According       According                                                     to the          to the                                                        invention       invention                                      ______________________________________                                                      a       b         c     d                                       Number of reducing                                                                          8       8        16    16                                       gas inlet ports                                                               Number of peripheral                                                                        3       8         6    16                                       gas outlet ports                                                              Blast volume(Nm.sup.3 /T)                                                                   582     574       830   812                                     Blast temperature                                                                          1097    1110      1100  1109                                     (°C)                                                                   Injected reducing                                                                           533     531       807   794                                     gas volume (Nm.sup.3 /T)                                                      Reducing gas tempe-                                                                        1181    1195      1205  1189                                     rature (°C)                                                            Recycling gas volume                                                                        247     249       354   360                                     (Nm.sup.3 /T)                                                                 Coke-oven gas volume                                                                        141     139       201   203                                     (Nm.sup.3 /T)                                                                          CO      39.9    42.6    35.6  41.9                                   Reducing gas                                                                           CO.sub.2                                                                               8.2     9.0     7.2   8.4                                   composition                                                                            H.sub.2 27.9    29.6    35.1  34.0                                    (%)     H.sub.2 O                                                                              6.0     4.2     7.0   4.8                                            N.sub.2 18.0    14.6    15.1  10.9                                            CO      16.2    17.6    18.7  19.6                                   Taken-out                                                                              CO.sub.2                                                                              19.7    16.5    16.5  14.9                                   gas composi-                                                                           H.sub.2 17.2    24.0    20.2  27.8                                   tion (%) H.sub.2 O                                                                             14.0    18.6    16.1  22.6                                            N.sub.2 32.9    23.3    28.5  15.1                                   Coke ratio (kg/T)                                                                          411     398       323   310                                      ______________________________________                                    

In Table 1, letters a and b show the operations with the separate typereducing gas distribution. In the case of b employing reducing gas inletports and eight peripheral gas outlet ports, as compared with theoperation a employing only three outlet ports, the amount of N₂ in thecomposition of the removed gas was lower by 9.6%, the amount of N₂ inthe reducing gas composition was lower by 3.4%, and the coke ratio waslower by 13 kg/T. It will thus be seen that the operation of b usingeight outlet ports along with eight reducing gas inlet ports, is moreeffective in reducing the coke ratio as compared with the operation ofa.

Letters c and d show the operations with the annular reducing gasdistribution. The reducing gas was introduced in an amount correspondingto the ratio U/(U+V) = 0.41. In the operation of c, 6 (six) peripheralgas outlet ports were used with 16 reducing gas inlet ports, while 16peripheral gas outlet ports were used in the operation of d. In theoperation of d, as compared with the case of c, the content of N₂ in thetaken-out gas composition was lower by 13.4%, the content of N₂ in thereducing gas was lower by 4.2%, and the coke ratio was lower by 22 kg/T.It will thus be seen that the operation of d employing 16 peripheral gasoutlet ports with 16 reducing gas inlet ports has a greater effect inreducing the coke ratio than the operation of c. Thus, it is evidentfrom the above Table 1 that the use of a number of peripheral gas outletports as determined according to equation (3) or (5) has a satisfactoryeffect in reducing the coke ratio of a blast furnace.

EXAMPLE 2:

The blast furnace plant shown in FIG. 7 was operated according to themethod of this invention. The peripheral gas outlet ports werepositioned according to the teachings of this invention, and thenitrogen content (5) in the peripheral gas taken out and the reducinggas injected were also controlled according to the teachings of thisinvention. The following Table 2 shows the comparison of the operationaccording to the invention and according to the conventional methods.

                  TABLE 2                                                         ______________________________________                                                       a       b         c                                            ______________________________________                                        U/(U+V)          0.38      0.38      0.38                                     U/n(U+V)          0.095     0.095     0.095                                   Θ (degree) --         0        10                                       Blast volume (Nm.sup.3 /T)                                                                      848       820       831                                     Blast temperature (°C)                                                                  1098      1101      1090                                     Injected reducing gas                                                                           760       758       761                                     volume (Nm.sup.3 /T)                                                          Reducing gas temperature                                                      (°C)      1200      1196      1215                                     Taken-out recycle gas                                                                           351       355       353                                     volume (Nm.sup.3 /T)                                                          Coke-oven gas volume                                                                            202       199       200                                     (Nm.sup.3 /T)                                                                            CO        27.2      42.3    34.7                                   Reducing gas                                                                             CO.sub.2   7.3       8.5     8.0                                   composition                                                                              H.sub.2   37.3      33.1    35.0                                   (%)        H.sub.2 O  6.7       4.7     7.2                                              N.sub.2   21.5      11.4    15.1                                              CO        20.1      19.4    19.0                                   Taken-out gas                                                                            CO.sub.2  17.5      15.4    16.5                                   composition                                                                              H.sub.2   11.0      27.1    19.1                                   (%)        H.sub.2 O 11.3      22.5    16.1                                              N.sub.2   40.1      15.6    29.3                                   Coke ratio (Kg/T)                                                                               335       308       321                                     ______________________________________                                    

In the above Table 2, letter a shows the operation according to aconventional method in which the top gas was nonselectively taken outand recycled; b the operation according to the method of this inventionin which the top gas was taken out and recycled from the peripheral gasstream just above the reducing gas inlet ports (θ = 0) (the N₂ contentwas the lowest); and c the operation in which the top gas was taken outand recycled from the gas stream outside the limits of the θ and thusoutside the range of this invention though it was a peripheral gasstream (the N₂ content was higher than in the case of c). In all ofthese operations, n (the number of reducing gas inlet ports) = 4.

It will be seen from the above Table 2 that in the operation where thereducing gas was injected in an amount corresponding to 38% of the totalquantity of the gas passed through the furnace shaft, the followingeffects were confirmed under the condition that about 200 Nm³ /T ofcoke-oven gas was used as a fresh feed gas for reforming purposes:

1. In the operation of a, the top gas taken out from the top of theshaft had an average composition and its N₂ content was as high as 40.1%so that the resultant reducing gas produced according to theabove-mentioned conditions would contain 21.5% N₂. In this case, thecoke ratio was 335 kg/T of pig iron. In other words, this operation isrepresentative of the conventional top gas recycling processes and itwas shown in this Table 2 as the standard of comparison.

2. The operation of b was performed in accordance with the method ofthis invention, and the nitrogen content in the taken-out recycling gaswas as low as 15.6%. Consequently, the N₂ content in the reducing gasreformed by using the above-mentioned amount of fresh feed gas(coke-oven gas) was 11.4% which was about one half of that in the caseof a. As a result, the resultant reducing gas contained CO ofparticularly high concentration with the result that the indirectreduction within the shaft of the blast furnace was facilitated and thecoke ratio was greately reduced to 308 kg/T.

3. In the operation of c, though the top gas was taken out from theperipheral gas stream, the position of the outlet ports was such thatthe value of θ was 10° which was outside the range of θ as definedaccording to this invention (in this case, the proper angle would beθ≧7°). Therefore, the taken-out recycling gas contained 29.3% N₂ andthus the resultant reformed reducing gas also contained 15.1% N₂.Consequently, the coke ratio was 321 kg/T.

A comparison between thhe operations a and b shows that the recyclinggas taken out from the peripheral gas stream contained considerablyreduced amount of N₂ and thus the nitrogen content in the resultantreformed reducing gas was considerably low. Thus, the coke ratioattained with the use of the peripheral gas taken out was lower by 27kg/T. It will thus be seen that the nitrogen content in a reducing gashas a detrimental effect on coke ratio and the nitrogen content in thereducing gas can be reduced with the use of the taken-out peripheralgas.

It will also be seen from a comparison between the operations b and cthat in the case of b the content of N₂ in the reducing gas was lower byabout 3.5%, the content of N₂ in the taken-out recycling gas was lowerby about 14%, and the coke ratio was lower by 13 kg/T as compared withthe case of c. This clearly shows the importance of controlling thevalue of θ within a range determined according to equation (6) and alsocontrolling the values of x and y within the ranges respectively definedaccording to equations (7) and (8).

We claim:
 1. In a method of blast furnace operation of the type in whicha reducing gas produced by reforming a mixture of a peripheral gasstream taken out from the top or shaft portion of a blast furnace and afresh feed materials is injected into the blast furnace through reducinggas inlet ports other than the ordinary tuyeres, wherein said peripheralgas stream is taken out through a plurality of outlet ports theimprovement comprising determining the number of said outlet ports inaccordance with either one of the following equations: ##EQU15## U/U+V =ratio of injected reducing gas quantity (U) to the total gas quantity(U+V) passing through furnaceV = quantity of gas produced from the blastby the combustion of coke and solution loss reaction m = number ofoutlet ports for peripheral gas stream n = number of inlet ports forreducing gas C/h = ratio by weight of C to H contained in a fresh feedgasand positioning said outlet ports for peripheral gas stream relativeto said inlet ports for reducing gas so that there is a predeterminedangular relation therebetween established in accordance with thefollowing equation: ##EQU16## where θ = angle made between verticalplane formed by peripheral gas stream outlet port and furnace centerline and vertical plane formed by reducing gas inlet port and furnacecenter line (in degrees) n = number of reducing gas inlet ports U =quantity of reducing gas injected V = quantity of gas produced fromblast by the combustion of coke and solution loss reaction
 2. A methodaccording to claim 1, including controlling the nitrogen content x (%N₂) of the gas taken out from the peripheral area of said furnace withina range established in accordance with the following equation: ##EQU17##where
 0. 1 ≦ U/(U+V) ≦0.9and controlling the nitrogen content y (% N₂)of the reducing gas to be injected within a range established inaccordance with the following equation: ##EQU18## where
 0. 1 ≦ U/(U+V) ≦0.9